Scenario: You want to change the explorer background of a folder on an Windows XP computer.
Step 1.
Open a command prompt and run the command attrib +s c:\test
Where C:\test should be replaced with the path to folder of your choice.
Step 2.
Browse to the folder, open the Tools menu, choose Folder options. Pick the tab View. Make sure Show hidden files are checked and Hide protected operating system files are unchecked.
Step 3.
You should now see a file called system.ini (otherwise, just create it using notepad).
Add the following lines to it.
[ExtShellFolderViews]
{BE098140-A513-11D0-A3A4-00C04FD706EC} = {BE098140-A513-11D0-A3A4-00C04FD706EC}[{BE098140-A513-11D0-A3A4-00C04FD706EC}]
Attributes=1
IconArea_Image=path_of_the_wallpaper
IconArea_Text=0x00000000
Save and close notepad. Close the folder and open it. The background should now have changed.